1. Field of the Invention
The invention relates to a ducted fan that produces thrust through the rotation of a fan arranged in a duct.
2. Description of the Related Art
Using a ducted fan as a thrust producing device for a vertical take-off and landing aircraft has been proposed.
Japanese Patent Application Publication No. JP-A-2005-145264 describes a vertical take-off and landing aircraft. In the described vertical take-off and landing aircraft, the magnitude of negative pressure, which is formed when air flows, is changed in a lip portion through which air is introduced into a duct of a ducted fan. Thus, the magnitude of a thrust produced is flexibly controlled.
Japanese Patent Application Publication No. JP-A-03-70699 also describes a vertical take-off and landing aircraft. According to Japanese Patent Application Publication No. JP-A-03-70699, the pitch angle of a blade of a ducted fan and the speed of an engine that drives the ducted fan are controlled. Thus, the magnitude of a thrust produced is flexibly controlled.
However, the following phenomenon may occur in a ducted fan mounted in an aircraft. If a crosswind blows against the side face of the ducted fan during flight, the airflow going into the duct and the crosswind join together at an air-introduction portion of a portion of the duct, the portion being exposed to the crosswind (hereinafter, such air-introduction portion will be referred to as a “front air-introduction portion”). As a result, the flow speed of the air entering the air-introduction portion increases.
Accordingly, a negative pressure formed at the front air-introduction portion is greater than a negative pressure formed in an air-introduction portion of another portion of the duct, the portion not being exposed to the cross wind (hereinafter, such air-introduction portion will be referred to as a “rear air-introduction portion”).
Accordingly, the front air-introduction portion is attracted in the direction, in which the negative pressure is applied, more strongly than the rear air-introduction portion is. Therefore, a moment, which is applied in the direction in which the front portion of the ducted fan moves upward, may be produced.
An airflow outlet is formed in the lower portion of the ducted fan. The airflow, which has passed through the duct, comes out of the airflow outlet. In an area downstream of the airflow outlet, the airflow coming out of the duct and the crosswind join together, and the direction of the airflow on the downstream end of the duct (hereinafter, such airflow will be referred to as the “descending airflow”) approaches the direction of the crosswind.
Because the direction of the descending airflow is influenced by the crosswind, the descending airflow proceeds through the area downstream of the duct so as to pass over the airflow outlet. This interrupts the flow path along which the airflow comes vertically out of the airflow outlet of the duct.
As a result, the air cannot smoothly flow out of the duct, which may decrease the flow speed of the airflow coming out of the duct. In particular, decreases in the flow speed of the airflow coming out of the rear airflow outlet decrease the thrust applied to the rear portion of the duct. This may make it difficult to produce a moment that is applied in the direction in which the rear portion of the duct moves upward.
Hereafter, the above-described phenomenon, which is caused when the ducted fan is exposed to the crosswind, will be described in detail with reference to FIGS. 1 and 2.
FIG. 1 illustrates the airflows that are generated around a ducted fan 200 and in a duct 7 when a crosswind 13 blows against the side face of the ducted fan 200 from the outside of the ducted fan 200. The arrows in FIG. 1 indicate the directions of airflows. To facilitate the understanding of the directions of the airflows, the fan provided in the duct 7 is not shown in FIG. 1.
First, the airflows near an airflow inlet 8 will be described with reference to FIG. 1. An airflow 14 generated due to the turning motion of the fan and the crosswind 13 join together in a front airflow inlet 8a to generate a combined airflow 18. The combined airflow proceeds faster than the airflow 14 flowing through a rear airflow inlet 8b. 
Next, the airflows near an airflow outlet 10 will be described with reference to FIG. 1. An airflow 20 coming out of the airflow outlet 10 and the crosswind 13 join together, which causes the direction of a descending airflow 19 to approach the direction of the crosswind 13.
Because the direction of the descending airflow 19 is influenced by the crosswind 13, the descending airflow 19 proceeds through the area downstream of the duct 7 so as to pass over the airflow outlet 10. This interrupts the flow path along which the airflow 20 comes vertically out of the airflow outlet 10.
As a result, the air does not flow smoothly out of the airflow outlet 10.
FIG. 2 illustrates the flow speed distribution of the airflows generated around the ducted fan 200 and in the duct 7, the directions and magnitudes of thrusts produced by the airflows, and the directions and the magnitudes of moments produced by the thrusts, in the case where the crosswind 13 blows against the side face of the ducted fan 200 from the outside of the ducted fan 200.
Due to the above-described airflows near the airflow inlet 8, the negative pressure formed at a ramped portion 9a, which is formed on the inner face of the front portion the duct 7 (hereinafter, referred to as a “front ramp portion 9a”), is greater than the negative pressure formed at a ramp portion 9b, which is formed in the inner face of the rear portion of the duct 7 (hereinafter, referred to as a “rear ramp portion 9b”). Accordingly, a force 15 for attracting the front ramp portion 9a is stronger than a force 16 for attracting the rear ramp portion 9b, whereby the front ramp portion 9a is attracted in the direction, in which the negative pressure is applied, more strongly than the rear ramp portion 9b is. Thus, a moment 17, which is applied in the direction in which the front ramp portion 9a moves upward, is produced.
A flow speed distribution area 21 is formed in the duct 7 and on the downstream end of the duct 7. In the flow speed distribution area 21, the airflows proceed at flow speeds equal to or higher than a predetermined value.
Due to such airflows, a thrust 22a is applied to the front portion of the duct 7 (hereinafter, such thrust will be referred to as a “front fan thrust”), and a thrust 22b is applied to the rear portion of the duct 7 (hereinafter, such thrust will be referred to as a “rear fan thrust”). Thus, a moment 23, which is applied in the direction in which the rear portion of the duct 7 moves upward, is produced.
As shown in FIG. 2, the moment 23 is not great enough to completely offset the moment 17, which is applied in the direction in which the front ramp portion 9a moves upward. Accordingly, if the ducted fan 200 is exposed to the crosswind 13, a moment, which is applied in the direction in which the front portion of the duct 7 moves upward (hereinafter, such moment will be referred to as a “pitch-up moment”), is ultimately produced.
As described so far, in the ducted fan according to the related art, the pitch-up moment, which is applied in the direction in which the front portion of the duct moves upward, may be produced when the ducted fan is exposed to the crosswind.